1. Field of the Invention
The present invention relates to a boot for covering a constant velocity joint which is indispensable as a joint for a drive shaft of a front-wheel-drive vehicle.
2. Description of the Related Art
Conventionally, a joint portion of a constant velocity joint is covered with a bellows-shaped boot with grease sealed therein, to maintain large-angle and smooth rotation by preventing the entry of water and dust. This constant velocity joint boot is comprised of a larger cylindrical portion having a large diameter and retained by a joint outer race or the like, a smaller cylindrical portion having a smaller diameter than the larger cylindrical portion and retained by a shaft, and a conical bellows portion for integrally connecting the smaller cylindrical portion and the larger cylindrical portion. During use, the bellows portion undergoes deformation in correspondence with a change in an angle (joint angle) formed by the shaft and the joint outer race or the like. Therefore, even if the angle becomes large, the joint portion can be reliably sealed by the boot.
This constant velocity joint boot was formerly formed of rubber in many cases. However, since there is a limit to durability in the case of rubber, a thermoplastic elastomer excelling weather resistance and fatigue resistance has come to be used in recent years. Meanwhile, in the case of the constant velocity joint boot, there is a need to impart a sealing function for reliably preventing the entry of water and dust into the joint. However, if the thermoplastic elastomer is used as the material, since there are many cases where the larger cylindrical portion is fitted in the form of different-shape fitting or non-circle fitting, and since the resiliency of the thermoplastic elastomer is high and its follow-up characteristic is not as good as rubber, there has been a problem in that it is difficult to ensure sealability. In addition, blow molding is convenient as a method of molding the constant velocity joint boot. Nevertheless, the surface of a mating member is generally noncircular, so that the boot needs to be provided with a shape corresponding thereto. However, in the case where such a boot is manufactured, with blow molding, it is difficult to form the shape of the inner peripheral surface of the larger cylindrical portion such that its sealability with respect to the mating member becomes high. In this aspect as well, it is difficult to ensure sealability.
Accordingly, JP-UM-A-02-087131 discloses a constant velocity joint boot in which the boot body is formed of a polyester-based thermoplastic elastomer, and a soft rubber-made annular grommet is inserted in its larger cylindrical portion. According to this constant velocity joint boot, even a grommet which has thick-walled portions and thin-walled portions and whose inner peripheral surface is noncircular can be manufactured with high accuracy by injection molding or the like. Therefore, the boot body may not necessarily have very high geometrical accuracy and can be manufactured by blow molding. Further, a tightening force based on a clamp is transmitted to the grommet through the larger cylindrical portion, and the grommet undergoes elastic deformation, thereby exhibiting the sealing function. Namely, the boot body is capable of ensuring durability, while the grommet is capable of ensuring sealability with respect to the mating member. In addition, since the boot body having a large shape as compared to the grommet can be manufactured by blow molding, man-hour can be reduced and the cost can be lowered.
The constant velocity joint boot thus made up of the boot body and the grommet is fitted to the mating member in a state in which the grommet is inserted in the larger cylindrical portion, and is subsequently tightened by a clamp from the outer peripheral side of the larger cylindrical portion. Accordingly, during the fitting to the mating member, the positioning of insertion becomes necessary. For this reason, conventionally, a recess is formed in an outer peripheral surface of the mating member, while a positioning projection is formed on an inner peripheral surface of the grommet, and positioning is effected by causing the positioning projection to engage the recess.
However, there has been a problem in that although the grommet is soft, since the hard larger cylindrical portion is present on its outer periphery, when the positioning projection is brought into sliding contact with the outer peripheral surface of the mating member during fitting to the mating member, the deformation in the diameter enlarging direction of the grommet is restricted, which constitutes resistance in fitting, so that the operational efficiency in fitting is poor. In addition, there are cases where an axial offset occurs between the larger cylindrical portion and the grommet. Further, there has been a drawback in that even if the positioning projection is engaged with the recess of the mating member during fitting, since the amount of deformation of the grommet is small, it is impossible to obtain a click feel of fitting, and it is impossible to confirm whether or not the positioning has been effected.
In addition, an annular sealing projection is formed on the inner periphery of the grommet, and its position in axial direction is located in the range of interference of the clamp. In addition, there are many cases where a sealing projection is also formed at a distal end of the positioning projection. However, during fitting to the mating member, since the sealing projections come into sliding contact with the mating member, wear and damage can occur in the sealing projections, in which case trouble occurs in sealability.
Accordingly, JP-UM-A-02-071122 discloses a constant velocity joint boot in which a slit extending in the axial direction from an end face of the larger cylindrical portion is formed, and a positioning protrusion is formed by avoiding that slit. If such an arrangement is provided, the enlargement of the diameter of the larger cylindrical portion is facilitated by the slit, and resistance during fitting can be reduced, but it is difficult to obtain a click feel at the time the positioning projection engages the recess. In the constant velocity joint boot made up of the boot body and the grommet, if consideration is given to the adoption of this structure, the formation of the slit in the grommet or the larger cylindrical portion is not recommendable from the viewpoint of sealability in addition to the problem of the click feel, and there is also the problem of increased man-hour.